Turbine engines can include a variety of systems to minimize undesired emissions, such as oxides of nitrogen (NOx) and carbon monoxide (CO), generated during combustion. For example, some turbine engines use catalytic combustors to reduce the generation of NOx because catalytic combustion occurs at temperatures well below the temperature necessary for NOx production (above about 2700 degrees Fahrenheit). Catalytic combustion can occur at temperatures up to about 1000 degrees Fahrenheit to about 1400 degrees Fahrenheit. In contrast, conventional combustion temperatures can range from about 2500 to about 2900 degrees Fahrenheit.
While successful in combating NOx emissions, catalytic combustors can result in higher CO output from the engine, even at desired operating points such as base load. The increase in CO levels can be partly attributed to the lack of flame propagation in the relatively laminar or quiescent flow exiting the catalytic modules. The higher CO emissions are also a symptom of the relatively cold temperatures at which catalytic combustion occurs. The amount of time needed for CO burnout is largely a function of temperature. Because the temperature of the catalytic combustion reaction is relatively low, catalytic systems typically need more time to burn the CO. The longer burnout time in turn requires a combustion system having a sufficiently long travel path for the combustion gases to provide the requisite time for burnout before the gases leave the combustion system; however, due to packaging issues and economic feasibility, many engines cannot accommodate extra physical length. Thus, burnout takes longer than the distance (and travel time) available. Further, once they impinge on the first row of vanes in the turbine section, the combustion gases are quenched to about 400 degrees Fahrenheit, below the temperature needed to achieve CO burnout. Consequently, the unburned CO is passed in the engine exhaust.
The above problems can arise in almost any type of catalytic combustor system, including both lean and rich catalytic systems. Thus, there is a need for a turbine engine having a catalytic combustor system that is configured to reduce CO emissions.